How do I round an edge of a rectangle?
You can use the Bevel tool.

In Edit mode, select all the edges you want to be rounded.
Hit Ctrl+Shift+B to initiate the tool.
Adjust the offset (radius) of the rounded corners by sliding your mouse.
Adjust the number of segments (subdivisions) you want with the mouse wheel. The more there is, the smoother the corner will be.
Validate the operation with a mouse click or by pressing Enter.

You can adjust those parameters, and many others, by expanding the &quot;Last Operator&quot; panel (bottom left of the Viewport; it should be labelled &quot;Bevel&quot;):

I assume you want to round edges of a rectangle, a planar surface; if you want to round the edges of a volume, you should use Ctrl+B instead.